A model for image formation in X-ray radiography

A model for x-ray radiographic image formation was developed and evaluated for its effectiveness in making spatial measurements of axially symmetric objects from digitized radiographs. An ideal image was first determined from geometric considerations using the exponential attenu-ation principle for x-radiation passing through matter. This image then served as the input signal to a linear system which simulates the image degradation due to penumbra, intensifying screen-film combination and scatter radiation effects. Recording on photographic film was modeled from a determination of film characteristics of a given radiograph.

Comparisons between a one-dimensional theoretical image calculated from the model and 200 separate radial scan lines from a digitized radiograph of a hollow plastic cylinder yielded a mean normalized cross-correlation of 0.9984. By comparing images calculated with different values for the inner hole radius of the cylinder to each scan line of the radiograph, an estimate of the radius was made. The measurement made in this manner determined the inner hole radius to within 119 microns of the machinist's value of 1.778 cm.